PURPOSE: To measure a particle velocity for every particle size by photodetecting a reflected ray containing plural colors obtained by illuminating colored particle group in a solid and gas mixture with a white ray, at two points spaced a specified distance, spectrally dividing it by a prism, and deriving a mutual correlation coefficient of an intensity signal of light of each color.

CONSTITUTION: Light from a white color light source 12 is transmitted by an illuminating optical fiber 3, particle groups 1-a, 1-b are irradiated simultaneously, its reflected ray is transmitted to prisms 6, 7 through photodetecting fibers 4, 5, and it is split into red lights 6-a, 7-a an blue lights 6-b, 7-b. Thereafter, they are made incident to optical-electric transducers 8, 9 or 10, 11, voltage being in proportion to intensity of light is generated, and red output volages 9, 8 and blue output voltages 11, 10 are inputted to correlators 13, 14, respectively. A mutual correlation coefficient of an input signal is calculated by the mutual correlators 13, 14, and a time delay quantity t1' whose coefficient becomes maximum is outputted. In this way, a particle velocity V1=l/t1' can be derived. Provided that l and V1 denote an interval of the photodetecting optical fibers 4, 5, and a particle velocity obtained by using red light.